Fire arm cartridge casings are typically made from brass. There are a few notable exceptions. Shotgun cartridge casings today are made from a combination of brass and plastic. In the past, shotgun cartridge casings having been made from brass and paper, brass and plastic, all brass and all plastic. Also, some pistol and rifle cartridges have been made from steel.
There are many reasons why cartridge casings made from aluminum would be highly desirable. Aluminum casing weights would be typically ⅓ the weight of either a steel or brass casing. For example four fully loaded aluminum cased cartridges weigh the same as three with brass or steel casings. This has significant impact on the battlefield where weight is critical—for either allowing aircraft to carry more ammunition, fuel or cargo, or relieving the load on a foot soldier, allowing him or her to go further faster, or carry more ammunition.
Another significant advantage aluminum has over brass is material pricing. Brass prices are closely coupled to copper prices which are typically much higher than aluminum.
Aluminum is also less harmful to the environment than the copper and lead in brass. Aluminum also has an advantage over steel in corrosion resistance.
While there are some pistol cartridge casings that are made from aluminum, as well as some cartridge casings for large projectiles (such as grenade launchers) made from aluminum, there are almost no aluminum cartridge casings for rifles.
There are several reasons why cartridge casings made for rifles have hither to not been made from aluminum.                1) Chamber pressures for different firearms are typically as follows: Rifle 60,000 psi, Pistol 40,000 psi, Shotgun 10,000 psi, Grenade Launchers 3000 psi. As the chamber pressure increases, stress on the casing increases. Thus, with higher pressures, damage from leaking propellant gasses can cause significantly more damage. Only aluminum alloys with exception tensile strength can safely withstand the chamber pressures of most rifle cartridges.        2) Aluminum has the capability of being explosive and in rapid high temperature ignition “explosive” environment, can become molten, and rapidly decompose (explosive plasma). Leakage of propellant past the primer in cartridge casings can cause the aluminum to melt and explode damaging the firearm and can cause injury. The explosive plasma nature of aluminum can cause any gas leak to have the potential to damage the firearm and cause significant injury.        3) Most rifle cartridge casings are “necked” meaning the end that secures the bullet is smaller than the end that holds the powder charge and primer. Such “necking” of rifles casings is highly desired to provide sufficient room for the required propellant without having to construct the cartridge with excessive length. Necking forces the casing to be made in one of two ways. Either the inside of the casing is machined away with a tool that has to be small enough to pass through the neck, then cut the inside diameter larger—a tedious time-consuming process referred to as “undercutting.” Alternatively, the neck may be somehow formed by other mechanical means. However, the formability of high-strength aluminum alloys is quite poor, making undercutting and other mechanical means difficult and expensive procedures.        
Accordingly, there is a need for an aluminum rifle cartridge casing which avoids the aforementioned problems in the prior art.